Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 13Chapter 13

Meiosis and Sexual Life Cycles


Overview: Hereditary Similarity and Variation

• Living organisms are distinguished by their ability to reproduce their own kind

• Heredity is the transmission of traits from one generation to the next

• Variation shows that offspring differ in appearance from parents and siblings

• Genetics is the scientific study of heredity and variation


Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

• In a literal sense, children do not inherit particular physical traits from their parents

• It is genes that are actually inherited


Inheritance of Genes

• Genes are the units of heredity

• Genes are segments of DNA

• Each gene has a specific locus on a certain chromosome

• One set of chromosomes is inherited from each parent

• Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation


Comparison of Asexual and Sexual Reproduction

• In asexual reproduction, one parent produces genetically identical offspring by mitosis

• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

Video: Hydra BuddingVideo: Hydra Budding


Concept 13.2: Fertilization and meiosis alternate in sexual life cycles

• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism


Sets of Chromosomes in Human Cells

• Each human somatic cell (any cell other than a gamete) has 46 chromosomes arranged in pairs

• A karyotype is an ordered display of the pairs of chromosomes from a cell

• The two chromosomes in each pair are called homologous chromosomes, or homologues

• Both chromosomes in a pair carry genes controlling the same inherited characteristics

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5 µmPair of homologouschromosomes

Sisterchromatids

Centromere


• The sex chromosomes are called X and Y

• Human females have a homologous pair of X chromosomes (XX)

• Human males have one X and one Y chromosome

• The 22 pairs of chromosomes that do not determine sex are called autosomes


• Each pair of homologous chromosomes includes one chromosome from each parent

• The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father

• The number of chromosomes in a single set is represented by n

• A cell with two sets is called diploid (2n)

• For humans, the diploid number is 46 (2n = 46)


• In a cell in which DNA synthesis has occurred, each chromosome is replicated

• Each replicated chromosome consists of two identical sister chromatids

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Key

Maternal set ofchromosomes (n = 3)

2n = 6

Paternal set ofchromosomes (n = 3)

Two sister chromatidsof one replicatedchromosomes

Two nonsister chromatids in a homologous pair

Pair of homologouschromosomes(one from each set)

Centromere


• Gametes are haploid cells, containing only one set of chromosomes

• For humans, the haploid number is 23 (n = 23)

• Each set of 23 consists of 22 autosomes and a single sex chromosome

• In an unfertilized egg (ovum), the sex chromosome is X

• In a sperm cell, the sex chromosome may be either X or Y


Behavior of Chromosome Sets in the Human Life Cycle

• At sexual maturity, the ovaries and testes produce haploid gametes

• Gametes are the only types of human cells produced by meiosis, rather than mitosis

• Meiosis results in one set of chromosomes in each gamete

• Fertilization, the fusing of gametes, restores the diploid condition, forming a zygote

• The diploid zygote develops into an adult


The Variety of Sexual Life Cycles

• The alternation of meiosis and fertilization is common to all organisms that reproduce sexually

• The three main types of sexual life cycles differ in the timing of meiosis and fertilization


• In animals, meiosis produces gametes, which undergo no further cell division before fertilization

• Gametes are the only haploid cells in animals

• Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism

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Key

HaploidDiploid

Gametesn

Diploidmulticellularorganism(sporophyte)

MitosisDiploidmulticellular

organism

FERTILIZATIONMEIOSIS

Zygote

n

n

2n 2n

Animals Plants and some algae Most fungi and some protists

n n

n

n n

nn

n

n

nFERTILIZATION

FERTILIZATION

MEIOSIS

MEIOSIS

Gametes Gametes

Zygote

ZygoteMitosis

Mitosis Mitosis Mitosis Mitosis

2n2n

2n

Spores

Haploid multicellular organism (gametophyte)

Haploid multicellular organism


• Plants and some algae exhibit an alternation of generations

• This life cycle includes two multicellular generations or stages: one diploid and one haploid

• The diploid organism, the sporophyte, makes haploid spores by meiosis

• Each spore grows by mitosis into a haploid organism called a gametophyte

• A gametophyte makes haploid gametes by mitosis


• In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage

• The zygote produces haploid cells by meiosis

• Each haploid cell grows by mitosis into a haploid multicellular organism

• The haploid adult produces gametes by mitosis


• Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

• However, only diploid cells can undergo meiosis

• In all three life cycles, chromosome halving and doubling contribute to genetic variation in offspring


Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid

• Like mitosis, meiosis is preceded by the replication of chromosomes

• Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

• The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

• Each daughter cell has only half as many chromosomes as the parent cell


The Stages of Meiosis

• In the first cell division (meiosis I), homologous chromosomes separate

• Meiosis I results in two haploid daughter cells with replicated chromosomes

• In the second cell division (meiosis II), sister chromatids separate

• Meiosis II results in four haploid daughter cells with unreplicated chromosomes

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Homologous pairof chromosomesin diploid parent cell

Interphase

Homologous pair of replicated chromosomes

Chromosomesreplicate

Meiosis I

Diploid cell withreplicatedchromosomes

Sisterchromatids

Meiosis II

Homologouschromosomesseparate

Sister chromatidsseparate

Haploid cells withreplicated chromosomes

Haploid cells with unreplicated chromosomes


• Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids

• The sister chromatids are genetically identical and joined at the centromere

• The single centrosome replicates, forming two centrosomes


Prophase I

• Prophase I typically occupies more than 90% of the time required for meiosis

• Chromosomes begin to condense

• In synapsis, homologous chromosomes loosely pair up, aligned gene by gene

• In crossing over, nonsister chromatids exchange DNA segments

• Each pair of chromosomes forms a tetrad, a group of four chromatids

• Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred


Metaphase I

• At metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole

• Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

• Microtubules from the other pole are attached to the kinetochore of the other chromosome

Animation: Metaphase I


Anaphase I

• In anaphase I, pairs of homologous chromosomes separate

• One chromosome moves toward each pole, guided by the spindle apparatus

• Sister chromatids remain attached at the centromere and move as one unit toward the pole

Animation: Anaphase I


Telophase I and Cytokinesis

• In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

• In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms

• No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Animation: Telophase I and Cytokinesis


Prophase II

• Meiosis II is very similar to mitosis

• In prophase II, a spindle apparatus forms

• In late prophase II (not shown in the art), chromosomes (each still composed of two chromatids) move toward the metaphase plate

Animation: Prophase IIAnimation: Prophase II


Metaphase II

• At metaphase II, the sister chromatids are arranged at the metaphase plate

• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

• The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Animation: Metaphase IIAnimation: Metaphase II


Anaphase II

• At anaphase II, the sister chromatids separate

• The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

Animation: Anaphase IIAnimation: Anaphase II


Telophase II and Cytokinesis

• In telophase II, the chromosomes arrive at opposite poles

• Nuclei form, and the chromosomes begin decondensing

• Cytokinesis separates the cytoplasm

• At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell

Animation: Telophase II and CytokinesisAnimation: Telophase II and Cytokinesis


A Comparison of Mitosis and Meiosis

• Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell

• Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

• The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis


• Three events are unique to meiosis, and all three occur in meiosis l:

– Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are carried to opposite poles of the cell

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Propase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

2n = 6

Parent cell(before chromosome replication)

Chromosomereplication

MITOSIS MEIOSIS

Chiasma (site ofcrossing over) MEIOSIS I

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Tetradspositioned at themetaphase plate

Metaphase IChromosomes positioned at themetaphase plate

Metaphase

AnaphaseTelophase

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Sister chromatidsseparate duringanaphase

Daughtercells of

meiosis I

Haploidn = 3

Anaphase ITelophase I

MEIOSIS II

Daughter cellsof mitosis

2n2n

n

Sister chromatids separate during anaphase II

n n n

Daughter cells of meiosis II


Property Mitosis Meiosis

DNA replication

During interphase

During interphase

Divisions One Two

Synapsis and crossing over

Do not occur Form tetrads in prophase I

Daughter cells, genetic composition

Two diploid, identical to parent cell

Four haploid, different from parent cell and each other

Role in animal body

Produces cells for growth and tissue repair

Produces gametes


Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

• Mutations (changes in an organism’s DNA) are the original source of genetic diversity

• Mutations create different versions of genes

• Reshuffling of different versions of genes during sexual reproduction produces genetic variation


Origins of Genetic Variation Among Offspring

• The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

• Three mechanisms contribute to genetic variation:

– Independent assortment of chromosomes

– Crossing over

– Random fertilization


Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

• In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

• The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

• For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

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Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1 Possibility 2

Combination 2Combination 1 Combination 3 Combination 4

Daughtercells

Metaphase II

Two equally probablearrangements ofchromosomes at

metaphase I


Crossing Over

• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent

• Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene

• In crossing over, homologous portions of two nonsister chromatids trade places

• Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

Animation: Genetic VariationAnimation: Genetic Variation

LE 13-11LE 13-11Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

Recombinantchromosomes

Metaphase I

Metaphase II

Daughtercells


Random Fertilization

• Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

• The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations

• Crossing over adds even more variation

• Each zygote has a unique genetic identity


Evolutionary Significance of Genetic Variation Within Populations

• Natural selection results in accumulation of genetic variations favored by the environment

• Sexual reproduction contributes to the genetic variation in a population, which ultimately results from mutations

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece.

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